1. Field of the Invention
The present invention relates generally to the field of a tracking control during the recording or reproducing of an optical disc, and more particularly to a system and method for detecting a tracking error signal to perform the tracking control.
2. Description of the Related Art
Recently, a rapid progress of informational society has caused an enormous amount of information to be processed. There has been a demand for the record media, which have so large storage capacity as to record and reproduce a large amount of information, have a small size and an excellent portability, as well as a rapid access time, to cope with such circumstances. The record media, which cope with the above-mentioned demand at this stage, include a magnetic tape, a magnetic disc, an optical disc, a magneto-optical disc, a semiconductor memory and the like. The magnetic tape excels in storage capacity, but has a low accessibility, thus being unsuitable for the use as currently demanded in which the rapid information processing is required. The semiconductor memory excels in rapid response, but is expensive, thus making it unsuitable to use such a memory in large quantities, for example in case where such semiconductor memories are used as an element having several tens of gigabyte.
On the contrary, the optical disc, which has a high accessibility, an excellent portability and a low price per bit is worthy of special remark. In addition, there has been pursued the development of technical matters to improve further the storage capacity and the recording density. There has also been conducted various research and development of devices to be used, recording formats, material for forming recording medium and a structure of the recording medium.
Providing the optical disc with the higher recording density and decreasing the optical wavelength to be used cause a demand for further improvement in structure of an optical pickup for recording information on the optical disc and reproducing the information, as well as in method for controlling the optical pickup. In such circumstances, there arise a need to consider also a method for making a positional control of a beam spot for recording information on the optical disc and reproducing the information.
Such a positional control of the beam spot requires two elements, i.e., a focus servo for collecting light on a recording surface of the optical disc and a tracking servo for causing a light beam to follow a circumferential recording track or recording pit. With respect to error detecting systems for controlling these two elements, an astigmatism system has usually been applied for the focus servo, and a phase difference system and a push-pull system for the tracking servo.
The tracking error detection through the phase difference system, which utilizes reproduction signals of pits as recorded in correspondence with information, making it impossible to obtain any tracking error signal on a disc with a groove, which has not as yet been subjected to a recording step. The push-pull system, in which the signal level depends upon an amount of reflected light from the optical disc, cannot provide a sufficient error signal in an optical disc such as a multi-layer disc having a small amount of reflected light. In view of such circumstances, it has been necessary to provide not only the error detection device utilizing the phase difference system, which is to be used during the reproduction step, but also the additional error detection device utilizing the push-pull system, which is to be used during the recording step, on the side of the recording and reproducing apparatus. As a result, a photo detecting element having four divided sections had to be used in order to cope with the above-mentioned requirements. In addition, the multi-layer structure does not provide a sufficient amount of reflected light and only the push-pull system and the phase difference system are incapable of coping with the disc having a groove, which has not as yet been subjected to a recording step, thus causing technical problems.